Add self.retry_after to SparkRateLimitError class

If one sets the wait_on_rate_limit to True in their CiscoSparkAPI
class instantiation, they will hit an error telling them that
retry_after is not an attribute of the SparkRateLimitError class.

This is because the RestSession class checks for its existence
when it catches the error at line 282 in restsession.py

Error:
  File "/home/daspano/../ciscosparkapi/restsession.py", line 282, in request
    if self.wait_on_rate_limit and e.retry_after:
AttributeError: 'SparkRateLimitError' object has no attribute 'retry_after'

This commit adds that the retry_after attribute as an int, as my testing
shows the Retry-After result always being an integer in seconds.

Ex:
-----------------------------------Response------------------------------------
429 Too Many Requests
Date: Wed, 15 Nov 2017 21:47:00 GMT
Content-Type: application/json
Content-Length: 182
Connection: keep-alive
TrackingID: ROUTER_5A0CB5D2-B92F-01BB-7C3F-AC12D9227C3F
Cache-Control: no-cache
Retry-After: 194
Server: Redacted
l5d-success-class: 1.0
Via: 1.1 linkerd
content-encoding: gzip
Strict-Transport-Security: max-age=63072000; includeSubDomains; preload

Author: sardoodledom

ciscosparkapi/exceptions.py

@@ -140,4 +140,12 @@ def __init__(self, response):
 
 class SparkRateLimitError(SparkApiError):
     """Cisco Spark Rate-Limit exceeded Error."""
-    pass
+
+    def __init__(self, response):
+        assert isinstance(response, requests.Response)
+
+        # Set a sane default just incase Spark lets us down
+        self.retry_after = 200
+
+        if 'Retry-After' in response.headers.keys():
+            self.retry_after = int(response.headers['Retry-After'])

ciscosparkapi/restsession.py

@@ -281,7 +281,7 @@ def request(self, method, url, erc, **kwargs):
                 # Wait and retry if automatic rate-limit handling is enabled
                 if self.wait_on_rate_limit and e.retry_after:
                     logger.info("Received rate-limit message; "
-                                "waiting {:0.0f} seconds."
+                                "waiting {0} seconds."
                                 "".format(e.retry_after))
                     time.sleep(e.retry_after)
                     continue